The objective of this study is to determine the potential human health hazard of exposures to magnetic fields associated with their generation, transport, and use of 50/60 Hz electricity. Phase 1 comprises only those work activities associated with procurement of equipment and materials needed for the construction/renovation of the exposure and monitoring systems and the developmental effort needed to determine that the system functions appropriately and meet the specifications. Phase 2 involves the conduct of a combination 13- and 26-week study. The 13-week study will be to replicate, as nearly as possible, the series of studies reported by Loscher, that 50 Hz magnetic fields have the potential to increase the breast cancer rate in (DMBA) treated female Sprague Dawley rats, The purpose of the 26-week study will be to determine whether 50 or 60 Hz magnetic fields can promote cancer in a standard DMBA model.